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STOCKHOLDER'S EQUITY (Details) (USD $) | 1 Months Ended | 12 Months Ended |  | 12 Months Ended |  | 0 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
May 31, 2013 | Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2011 | Jun. 30, 2013 | Sep. 30, 2013 Net Unrealized Gains (Losses) on Cash Flow Hedges [Member] | Sep. 30, 2012 Net Unrealized Gains (Losses) on Cash Flow Hedges [Member] | Sep. 30, 2013 Defined Benefit Pension and Other Postretirement Benefit Plans [Member] | Sep. 30, 2012 Defined Benefit Pension and Other Postretirement Benefit Plans [Member] | Nov. 26, 2013 Subsequent Event [Member] | Aug. 13, 2013 Laclede Gas [Member] | Sep. 30, 2013 Laclede Gas [Member] | Nov. 26, 2013 Laclede Gas [Member] Subsequent Event [Member]
Accumulated other comprehensive income (loss), net of income taxes [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, issued (in shares) |  | 32,700,000 | 22,540,000 |  |  |  |  |  |  |  |  |  |
Increase in common stock and paid-in capital |  | $ 435,800,000 | $ 5,000,000 |  |  |  |  |  |  |  |  |  |
Shares issued - dividend reinvestment plan and stock purchase plan (in shares) |  | 44,074 | 46,107 |  |  |  |  |  |  |  |  |  |
Shares issued - dividend reinvestment plan and stock purchase plan |  | 1,847,000 | 1,841,000 | 1,614,000 |  |  |  |  |  |  |  |  |
Shares issued - Equity plan and stock-based compensation costs |  | 434,000,000 | 3,200,000 |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Share-based Compensation, Gross | 10,005,000 |  |  |  |  |  |  |  |  |  |  |  |
Shares issued - Equity plan and stock-based compensation costs (in shares) |  | 108,331 | 62,590 |  |  |  |  |  |  |  |  |  |
Dividend payment restriction formula component - fixed amount |  | 8,000,000 |  |  |  |  |  |  |  |  |  |  |
Dividend payment restriction formula resulting calculation - available to pay dividends in period |  | 833,000,000 | 355,000,000 |  |  |  |  |  |  |  |  |  |
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares) |  | 285,222 |  |  |  |  |  |  |  |  |  |  |
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares) |  | 194,246 |  |  |  |  |  |  |  | 186,919 |  |  |
Issuance of first mortgage bonds |  | 550,000,000 | 0 | 0 |  |  |  |  |  |  | 450,000,000 |  |
MoPSC authorized debt issuance through June 30, 2013 - maximum available |  |  |  |  | 518,000,000 |  |  |  |  |  |  | 518,000,000 |
Extension period for debt securities and preferred stock issuance authority |  | 2 years |  |  |  |  |  |  |  |  |  | 2 years |
MoPSC authorized debt issuance - remaining available |  | 370,800,000 |  |  |  |  |  |  |  |  |  |  | 370,800,000
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Balance, beginning balance |  | (4,116,000) | (2,100,000) |  |  | (1,886,000) | 320,000 | (2,230,000) | (2,420,000) |  |  |  |
Current-period change |  | 3,329,000 | (2,016,000) | 5,037,000 |  | 3,281,000 | (2,206,000) | 48,000 | 190,000 |  |  |  |
Balance, ending balance |  | $ (787,000) | $ (4,116,000) | $ (2,100,000) |  | $ 1,395,000 | $ (1,886,000) | $ (2,182,000) | $ (2,230,000) |  |  |  |

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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e681-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 + Details
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